Electrons trapped in high earth orbits and electrons and protons trapped in low and medium earth orbits cause a high level of ionizing radiation in space. Such ionizing radiation causes an accumulation of charge in electronic circuits which eventually results in a malfunction or failure of the circuits.
Electron-hole pairs generated in the bulk silicon of an integrated circuit do not present a severe problem, as the electrons and holes recombine rapidly. Electron-hole pairs formed near the field oxide of an integrated circuit are more difficult to deal with because the electrons are far more mobile than the holes and may become separated from the holes and trapped near the field oxide interface. This interferes with recombination and results in an accumulation of net positive charge in the field oxide, or other dielectric film. The edge region between the diffusion region and the field oxide below a polysilicon gate, referred to as the “bird's beak” region, is particularly susceptible to the effect of the ionizing radiation. The accumulation of net positive charge in the field oxide beneath the polysilicon gate can cause leakage of electrons across the gate, turning on the gate prematurely. Even slight leakage across the many gates in a typical integrated circuit can cause excess power drain and overheating of the integrated circuit.
Integrated circuit designs have been developed to withstand high levels of ionizing radiation. Such design methodologies can involve redundancy of electronic circuits, suitable doping of the semiconductor material and spacing of electronic circuits. Such methodologies require increased cost for redesign and production.
Typical NMOS transistors 100 and 102 are shown in FIG. 1. Transistor 100 includes source/drain regions 104 and 108, and polysilicon gate 106. Transistor 102 includes source/drain regions 112 and 114, and polysilicon gate 116. If one of the source/drain contacts of transistor 100 is coupled to ground as shown, and the adjacent source/drain contact of transistor 102 is coupled to VCC as shown, then inter-device leakage 110 can occur between the two transistors due to the presence of ionizing radiation. In addition, intra-device leakage 118 can also occur between source/drains 112 and 114, if one of the source/drain contacts is coupled to ground, and the other is coupled to VCC, as shown.
An N-channel transistor circuit 200 is shown in FIG. 2A. Transistor circuit 200 includes two N-channel transistors coupled together, suitable for use in either a NAND or NOR gate. Transistor circuit 200 includes a first transistor M1 having a source/drain 202, and a gate 204. Transistor circuit 200 also includes a second transistor M2 having a source/drain 214, and a gate 210. The other source/drains of transistors M1 and M2 are coupled together at node 208. Body contacts 206 and 212 can be coupled to ground. In a NAND gate 220, source/drain 202 is coupled to two P-channel transistors as shown in FIG. 2B and source/drain 214 is coupled to ground. In a NOR gate 230, source/drains 202 and 214 are coupled to ground, and node 208 is coupled to two P-channel transistors as shown in FIG. 2C.
The N-channel transistor circuit 200 is susceptible to intra-device and inter-device leakage currents due to ionizing radiation, just as is a single N-channel transistor.
One prior art technique for forming a radiation-hardened transistor circuit 200 is shown in FIG. 3. Two annular transistor circuits are shown, each containing two N-channel transistors as is taught in U.S. Pat. No. 6,570,234 to Gardner, which is hereby incorporated by this reference. A first transistor circuit device 300 includes source/drains regions 308, 306, and 304 corresponding to source/drain regions S/D 1, S/D 2, and S/D 3 shown in FIG. 2. Transistor circuit 300 also includes first and second annular gates 302 and 310, as well as a thick field oxide region 312. A second transistor circuit device 314 includes source/drains regions 324, 322, and 320 corresponding to source/drain regions S/D 1, S/D 2, and S/D 3 shown in FIG. 2. Transistor circuit 314 also includes first and second annular gates 318 and 326, as well as a thick field oxide region 328.
Transistor circuits 300 and 314 effectively reduce leakage current due to ionizing radiation. Inter-device leakage current in region 316 is effectively reduced if source/drain regions 304 and 320 are coupled to ground. Additionally, intra-device leakage current along edge 330 is effectively reduced since both halves of the annular gate “A” 318 are at the same potential.
While transistor circuits 300 and 314 (and other known annular transistor and transistor circuit designs known in the art) effectively reduce leakage currents induced by ionizing radiation, they do so at the expense of precious integrated circuit area. Annular gates have four sides, and therefore take up much more area than a standard gate such as the gates of the prior art transistors shown in FIG. 1.
What is desired, therefore, is a transistor architecture and transistor circuit device architecture that has the desirable radiation-hardened characteristics of annular designs, but does so in a much smaller area.